In the transmission of speech signals in a telephone network between an exchange and a telephone set, it is a known problem that a varying line current causes distorsion of the transmitted speech signal. The line current, which is a direct current, is therefore kept at a constant level by electrical circuits included in the speech circuit of the telephone set, e.g. as illustrated in an article "Alla telefonens functioner i ett chip, PL3780" by Bengt Berg in "Ericsson Review", No 3, 1985, pp 138-143. For the transmission of speech signals over long telephone lines with low line current there have been developed feed circuits, e.g. for the receiver in a telephone set, which enable the speech signal to be transmitted without distortion. Such a circuit has been described, e.g. in IEEE, Vol. CE-28, No 3, 22. August 1982, M. Yoshitoshi et al, "All-electric Telephone Set with Low-Voltage Operating Circuit and Piezoelectric Transducers", and in the U.S. Pat. No. 4,319,094 cited in this article. According to this patent, a constant portion of the line current is taken off to charge a constant voltage circuit. The energy from this circuit is utilized when the speech signal has a large amplitude and can be distorted due to the line voltage being low. The apparatus is relatively complicated and has the drawback that it continuously takes off the contant portion of the line current, ever when the constant voltage circuit is fully charged.